Fluorescent bands are produced in chromosome preparations by treatment with acridine and bibenzimidazole dyes owing to localized variations in DNA base composition. On the basis of cytological observation, a dAT polymer inferred to be present in the human Y-chromosome will be sought and characterized. The mapping of eucaryotic chromosomes at fluorescence (light) microscope level according to base composition, as inferred from dye binding studies, will be extended to the molecular level by partial denaturation mapping (also specific for region rich in AT base pairs) of the eucaryotic DNA fragments subsequent to clonal selection and amplification in bacterial cells. BIBLIOGRAPHIC REFERENCES: Tanaka, T., and Weisblum, B. Systematic difference in the methylation off ribosomal ribonucleic acid from gram-positive and gram-negative bacteria. J. Bacteriol. 123:771-774. (1975) Tanaka, T., and Weisblum, B. Construction and characterization of a colicin El-R factor composite plasmid in vitro: Means for amplification of deoxyribonucleic acid. J. Bacteriol. 121:354-362. (1975)